An acetic acid-sulfuric acid derivative of hematoporphyrin (HPd) has been used in tumor diagnosis since 1961. The metabolite concentrates in soluble portion of tumor cells as a general function of malignancy. Its fluorescence pattern is favorable to the detection of malignant tissue because the excitation peak is sharp and centered at 407 nm, while the fluorescence peaks are in the deep red spectrum. Use of this method in designation of malignancy in lesions of the posterior ocular pole has been blocked in the past by interfering fluorescence from the crystalline lens. This can be by-passed by optical systems in which axis of illumination is disparate from, but close to, that of observation. Both an indirect ophthalmoscopic and a biomicroscopic system employing such a principle are in use. In both cases, the clinical observation of characteristic red glow of tumor tissue is supported by objective measurement of characteristic fluorescence. For the latter, two simultaneous methods are in use: a video-display-recording-integrating system, and a fiber optic probe-photomultiplier strip-chart recording system. The methodology was found to be 94% accurate in investigation of an experimental tumor in rabbits. The method has been considerably refined and has now been developed for use in human tumor investigation. Patients suspected of harboring malignant lesions in the posterior ocular segment are being investigated. The only toxicity encountered is that of increased skin photosensitivity. The hazard is reduced by proper self-protection of patients who are being examined. In addition, the use of singlet oxygen scavengers is under investigation in a joint project with the Dermatology Department at Columbia-Presbyterian Medical Center. The type of lesion for which this HPd appears promising is a small malignant melanoma or metastatic lesion in the posterior pole, since in such a situation, the lesion is surgically poorly approachable for 32P study. The hematoporphyrin derivative identification system is non-invasive.